Method and apparatus for neutralizing space charge in an ion beam

ABSTRACT

Space charge effects in an ion implanter can be caused by the mutual repulsion of ions of a particular polarity in a beam of ions which tend to cause the beam to “blow up” and become uncontrollable. This occurs for example in the ion implanter along the path of the ion beam and in particular at regions of external electric field. Introducing into the ion beam a second polarity of ions space charge neutralises the ion beam.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/GB98/02032 which has an Internationalfiling date of Jul. 10, 1997 which designated the United States ofAmerica.

The present invention is concerned with neutralising space charge in ionbeams.

Ion beams may be used for a number of purposes. In particular, ions ofdesired dopant species can be implanted in semiconductor substrates. Socalled “ion implanters” produce a beam of ions of the required dopantspecies which is directed at the substrate to implant the ions into thesemiconductor material.

In the absence of any neutralising effect, a beam containing only ionsof a particular polarity will is experience space charge effects. Themutual repulsion of the ions in the beam tends to cause the beam to“blow up” and become uncontrollable. In regions of zero electric field,self neutralisation of ion beams tends to occur, through the productionof electrons resulting from collisions between beam ions and slow movingatoms of residual gas in the vacuum chamber through which the beam ispassing. However, in regions of electric field, for example when theions of the beam are being accelerated or decelerated, or the beam isbeing electrostatically deflected, neutralising electrons are quicklyremoved from the beam because of their high mobility.

It is therefore a problem to control ion beams and prevent beam blow up,especially in regions containing an external electric field. The effectof beam space charge is especially severe for relatively low energybeams, since for the same beam current, there is a higher density ofions in a low energy beam.

In machines, such as ion implanters, where it is required to present abeam having a predetermined energy onto a target, particular problemsarise where the ion beam is first extracted from the ion source, andelsewhere over the beam path where the beam energy is changed,particularly where the beam is decelerated prior to hitting the target.In the case of extraction of ions from an ion source, the need to obtaina usefully high beam current limits the minimum energy at which ions canbe extracted from the arc chamber of the source. The region between theexit window of the source arc chamber and the extraction electrode is aregion of substantial electric field in which electrons cannot exist forsignificant times. In the extreme, for high currents and low extractionenergies, the theoretical density of ions in the beam leaving the sourceproduces a beam potential similar to the potential on the extractionelectrode. Clearly in such a case, the ions in the beam will notexperience the required accelerating field and no beam is in factproduced. In fact, space charge effects can severely reduce theextraction efficiency from ion sources at low extraction energies.

In ion implanters, it has therefore been the usual practice to extractions from the ion source at no less than a minimum extraction energy,typically not less than 10 keV, and for good efficiency often at higherenergies. In order to present a beam of lower energy onto the targetsubstrate, the beam must be subsequently decelerated, and space chargeproblems then arise also in the region of the decelerating field.

There is accordingly a requirement for some mechanism for the efficientextraction of ions from an ion source at relatively low energies, andmore generally for arrangements for controlling and neutralising spacecharge effects in ion beams, especially in regions of externally appliedelectric field.

According to the present invention, a method of neutralising spacecharge in an ion beam comprising ions of a first polarity, comprises thesteps of generating ions of a second polarity, and introducing saidsecond polarity ions to space charge neutralise the ion beam.

The ions of second polarity should be ions of species which can betolerated in the ion beam process. For example in an ion implantationprocess the ions may be of atoms such as He, Ne, Ar, Kr or Xe, or ofmolecules such as N₂, CO₂, CO₂, CO, O₂, Cl₂, Br₂, I₂. In each instance,the selection of an appropriate species is dependent on the need toavoid unwanted reactions and effects in the beam process. For example O₂would not be suitable for neutralising an ion beam being extracted froman ion source employing a hot cathode, such as a Bernas source, as theO₂ would quickly corrode the cathode. However O₂ may be tolerated withR.F. or microwave energised ion sources.

For effective space charge neutralisation, the density of ions of thefirst polarity in a region of the ion beam should equal the density ofcharged particles of the opposite polarity, assuming both the ions andparticles are singly charged. However, the density of charged particlesin a beam is proportional to the current density of those particles inthe beam and inversely proportional to the velocity of the particles inthe beam direction. The velocity of charged particles, on the otherhand, is proportional to the square root of the energy of the particlesand inversely proportional to the square root of the mass of theparticles.

The overall effect is that singly charged particles of high mass willhave much lower velocities, for the same energy, than lighter particlesand especially when compared with electrons. Thus, beam space charge canbe effectively neutralised with opposite polarity ions in regions of thebeam where neutralisation by electrons may not be possible.

In order to reduce the current density of the second polarity ions inthe beam, in a region of applied electric field, required to maintainneutralisation, the mass of the second polarity ions should be as highas possible. Large molecules, including organic molecules may then beuseful, such as B₁₀H₁₂, C_(x)H_(y) (hydro-carbons) or C_(x)H_(y)OH(alcohols). Alternatively large cluster ions may be employed.

Preferably, the second polarity ions should have a mass/charge ratio ofat least 400.

In a preferred example, the method includes applying an externalelectric field to a region of the ion beam and introducing said secondpolarity ions in said region. As explained previously, beamneutralisation problems arise especially in regions of external electricfield.

Then said second polarity ions are accelerated by said external electricfield in a field gradient direction, and said second polarity ions maybe introduced at a location in said region which is upstream relative tosaid field gradient direction.

It is normal practice in ion beam machines where a positive ion beam isaccelerated or decelerated between a first electrode at a firstpotential and a second electrode at a second higher potential, toprovide an electron suppression electrode between the first and secondelectrodes at a potential which is more negative than that of the firstelectrode, to prevent electrons being drawn from the beam beyond thefirst electrode by the influence of the potential on the secondelectrode. Then, for such a positive ion beam, ions of negative polarityare preferably introduced adjacent to the electron suppressionelectrode.

The invention also provides apparatus for neutralising space charge inan ion beam, comprising means producing a beam of ions of a firstpolarity, means generating ions of a second polarity, and meansintroducing said second polarity ions to space charge neutralise the ionbeam.

Examples of the present invention will now be described with referenceto the accompanying drawing which is a schematic illustration of an ionsource for positive ions with an extraction arrangement incorporating anexample of the present invention.

Referring to the FIGURE, an ion source comprises an arc chamber 10 whichmay be configured as a Bernas source, or any other known ion source. Afeed gas, here shown as BF₃ is supplied to the arc chamber along a pipe11 and the arc chamber operates to create a plasma within the arcchamber in which are formed the ions B⁺and BF₂ ⁺, for example. Thesepositive ions are then extracted from the arc chamber via an exitaperture 12 to form a beam. In the described example, the source isproviding a beam of boron ions (or BF₂ ions) which may be useful forimplanting a silicon substrate with boron. However, other ions may beproduced where required, such as As⁺, P⁺or Ar⁺, for example.

The ions are extracted from the arc chamber 10 by the electric fieldsproduced by electrodes 13 and 14. To extract positive ions from thesource, the arc chamber 10 and in particular the front face 15containing the exit aperture 12 is held at a positive potential relativeto the electrodes 13 and 14. In practice, the electrode 14 may form theentrance aperture to a mass analysing magnet and will usually be atground potential. The mass analysing magnet and a subsequent massselection slit are used to select from the ions drawn from the arcchamber 10 ions of the precise mass required for implanting. Forexample, the mass analyser and mass selection slit may select onlyB⁺ions for onward transmission to the semiconductor substrate target.Neither the mass analyser magnet nor the subsequent elements of the beampath to the substrate target are illustrated in the drawing. Thesecomponents may be typical of those customarily used in this art.

In order to ensure that ions entering the mass analyser magnet, passingthrough the electrode 14, have a well defined energy, the potential ofthe front face 15 of the arc chamber 10 is controlled by a power supply16. In the illustrated example, power supply 16 applies a potential ofabout 2 kV or less to the front face, so that the energy of the beam inthe mass analyser will be correspondingly 2 keV or less depending on thepotential applied.

The electrode 13 constitutes an electron suppression electrode and isset by a power supply 17 at a negative potential relative to theelectrode 14 so as to prevent electrons in the beam downstream of theelectrode 14, in the direction of the arrow 18 from being drawn out ofthe beam by the positive potential on the arc chamber 10. In this way,space charge neutralisation within the analyser magnet, downstream ofthe electrode 14, is largely maintained.

In the described example, the potential on the suppression electrode 13is about −200 volts, but suppression potentials of several kilovoltsbelow the ground potential of the analyser magnet and electrode 14 maybe used.

As can be seen in the drawing, a substantial electric field existsbetween the exit aperture 12 of the arc chamber and the suppressionelectrode 13, and also, though to a lesser extent, between thesuppression electrode 13 and the ground electrode 14 at the entrance tothe mass analyser. In these regions, any electrons in the beam have veryshort residence times due to their small mass and high mobility. As aresult, space charge neutralisation by electrons in the beam in theseregions is ineffective.

In the described example, a source of argon gas is supplied along a pipe20 to expand in a chamber 21. The sudden expansion of the argon gas inthe chamber 21, causes clusters of argon atoms to condense together,producing clusters each of at least 100 atoms and in appropriateconditions, up to 1000 atoms or more.

Within the chamber 21, a heated cathode, 22 emits electrons, which arcaccelerated through a grid electrode 23. The cathode 22 is biasedrelative to the grid 23 by a power source 24 to produce electrons of lowenergy (below about 50 eV). The resulting “spray” of low energyelectrons passing through the grid 23 ionises argon clusters within thechamber 21, forming negatively charged cluster ions. The cluster ions inthe chamber 21 diffuse from the chamber through an aperture 25immediately adjacent the electron suppression electrode 13.

The resulting flood of negatively charged cluster ions emerging from theaperture 25 assists in neutralising the space charge of the portion ofthe ion beam between the exit aperture 12 of the arc chamber 10 and thesuppression electrode 13.

As explained previously, for total space charge neutralisation, thedensity in a particular region of the ion beam of positive ions shouldequal the density of negative cluster ions (N_(b)=N_(c)).

Further, N_(b)=J_(b)/ev_(b)=N_(c)=J_(c)/ev_(c), where

J_(b) is the current density resulting from the positive beam ions,

v_(b) is the velocity of those ions in the beam,

J_(c) is the current density in the beam of negative cluster ions, and

v_(c) is their velocity.

If at a particular location in the beam, the energy of the requiredpositive ions is equal to the energy of the negative cluster ions,

m_(b) v_(b) ²=m_(c) v_(c) ²,

where m_(b) is the mass of the positive ions and m_(c) is the mass ofthe cluster ions.

From the above, J_(b)/J_(c)=v_(b)/v_(c)=(m_(c)/m_(b)).

Cluster ions comprising between 200 and 300 argon atoms are typicallyformed in the chamber 21. However, taking a minimum of 100 atoms in acluster ion, m_(c)≧4000 a.u. If the positive ion in the ion beam isB⁺(mass≈10.8), m_(c)/m_(b)≈400 and J_(b)/J_(c)≈20.

Thus, for full space charge neutralisation in the region of the beamwhere both the positive ions and the negative cluster ions have the sameenergy, the current of cluster ions in the beam, accelerated by theelectric field towards the exit aperture 12 of the arc chamber 10, mustbe about one-twentieth of the beam current of boron ions from the arcchamber. For a typical boron beam current of 5 mA, this implies acluster ion current of 0.25 mA.

In the Figure, a power supply 26 is illustrated connected to apply anegative bias to the cluster ion source relative to the electronsuppression electrode 13. In fact, the cluster ion source may be held atthe same potential as the suppression electrode 13, relying on anyresidual positive charge in the ion beam to draw cluster ions from theaperture 25 and into the beam. However, a small negative bias mayadditionally be applied to the cluster ion source to control the flow ofcluster ions.

In the illustrated example, the cluster ion source is shown deliveringcluster ions on the upstream side of the suppression electrode. Sincethe potential difference between the arc chamber 10 and the suppressionelectrode 13 is likely to be greater than that between the suppressionelectrode 13 and the grounded electrode 14, the problem of space chargesuppression in this region is most severe, especially if it is desiredto extract relatively high currents at low energy from the arc chamber10. However, cluster ions may also be delivered on the other side of thesuppression electrode 13 to neutralise space charge of the ion beam inthe region between the suppression electrode 13 and the groundedelectrode 14.

The example of the invention described above refers to a positive ionbeam comprising boron ions. However, the invention is applicable equallyto other desired positive ion beams. The invention may also be appliedto beams of negative ions, in which case positive cluster ions areintroduced. Positive cluster ions may be formed in the chamber 21 byspraying the condensed clusters of atoms with electrons of higherenergy.

Further, the above examples describe using argon cluster ions. Othergases may be used which can be made to produce large clusters of atoms.For ion implantation purposes, the cluster ions should be of a specieswhich can be tolerated in the implantation process. Further, for minimummobility of the cluster ion in the electric field regions of the ionbeam, relatively heavy atoms are preferred such as xenon.

Also, the above described example refers to neutralising the ion beam atthe point of extraction from the arc chamber of the ion source. Examplesof the invention may be used also at other regions of a beam where anexternal electric field renders the lifetime of any electrons in thebeam extremely short so that space charge neutralisation becomes aproblem. For example, negative cluster ion neutralisation may beemployed in a region where the ion beam is accelerated, and moreparticularly decelerated, by means of an electric field prior to impacton a target.

Also cluster ions may be used to provide beam space chargeneutralisation in a region where a beam is scanned transversely. In thiscase, the cluster ion neutralisation process described may be usefuleven if the beam scanning is conducted by magnetic fields, rather thanelectric fields. In such regions, self neutralisation of the ion beam,by the creation of electrons through collisions with residual gas atoms,may be insufficient to maintain adequate control of the beam potential.The beam may be scanned too rapidly to allow sufficient numbers ofelectrons to accumulate in the beam to provide adequate neutralisation.Flooding the scanning region with massive negative cluster ions shouldsubstantially improve beam neutralisation.

Ions of opposite polarity may also be used for neutralising an ion beamat other locations along the beam between the ion beam source and theion beam process target. For example, such opposite polarity ions may beinjected into the ion beam containing volume of a magnet used for massanalysis (or energy analysis) of the process ion beam. Improved beamneutralisation and control may then be achieved in the magnet,especially at low beam energies and high beam currents. Oppositepolarity ions may also be used to neutralise an ion beam in so calleddrift regions of no electric or magnetic field. Examples of driftregions in an ion implanter are between the ion source extraction opticsand the entrance to the mass analysis magnet, between the mass analysismagnet and the mass resolving system, and between a post mass selectionacceleration (or deceleration) stage and a substrate neutralisationsystem.

The invention is not restricted to the use of cluster ions forneutralising the beam. Some improvement in beam control may be achievedwith ions of the second polarity (negative ions for a positive ionbeam). Even He has a mass 4 which is about 10⁴ times the rest mass of anelectron (˜5.5×10⁻⁴ a.u.), so that the current density of He⁻for thesame neutralising effect is only one hundredth that for electrons.Generally, the second polarity ions should be of a species which willnot have substantial deleterious effects in the process. More massiveions and cluster ions are preferred, especially for neutralising inregions of applied electric field.

What is claimed is:
 1. A method of neutralising space charge in an ionbeam comprising ions of a first polarity, the method comprising thesteps of generating ions of a second polarity, and introducing saidsecond polarity ions to space charge neutralise the ion beam, wherein anexternal electric field is applied to a region of the ion beam, and saidsecond polarity ions are introduced in said region.
 2. A method asclaimed in claim 1, wherein said second polarity ions are accelerated bysaid external electric field in a field gradient direction and saidsecond polarity ions are introduced at a location in said region whichis upstream in said field gradient direction.
 3. A method as claimed inclaim 2, wherein said second polarity ions have negative polarity andare introduced adjacent to an electron suppression electrode in the beamextraction system of a source of positive ions.
 4. A method as claimedin claim 1, wherein said second polarity ions are cluster ions.
 5. Amethod as claimed in claim 4, wherein said cluster ions have amass/charge ratio of at least
 400. 6. A method as claimed in claim 1,wherein the ion beam comprises positive ions and said second polarityions are negative.
 7. Apparatus for neutralising space charge in an ionbeam, comprising means producing a beam of ions of a first polarity,means applying an external electric field to a region of said beam,means generating ions of a second polarity, and means introducing saidsecond polarity ions in said region to space charge neutralise the ionbeam.
 8. Apparatus for neutralising space charge in an ion beam,comprising a source of ions of a first polarity, said source producing abeam of said first polarity ions, electrodes to apply an externalelectric field to a region of said beam, a chamber in which ions of asecond polarity are generated, and an aperture from said chamberadjacent to said region of said beam through which said second polarityions diffuse into said region to space charge neutralise said beam.
 9. Amethod of neutralising space charge in an ion beam comprising ions of afirst polarity, the method comprising the steps of generating ions of asecond polarity, and introducing said second polarity ions to spacecharge neutralise the ion beam, wherein said second polarity ions arecluster ions having a mass/charge ratio of at least
 400. 10. A method asclaimed in claim 9, including applying an external electric field to aregion of the ion beam, wherein said second polarity cluster ions areintroduced in said region.
 11. Apparatus for neutralising space chargein an ion beam, comprising a source of ions of a first polarity, saidsource producing a beam of said first polarity ions, a chamber in whichcluster ions of a second polarity are generated, said cluster ionshaving a mass/charge ratio of at least 400, and an aperture from saidchamber adjacent to said beam through which said second polarity ionsdiffuse to space charge neutralise said beam.
 12. A method ofneutralising space charge in an ion beam comprising ions of a firstpolarity, the method comprising the steps of generating ions of a secondpolarity, and introducing said second polarity ions to space chargeneutralise the ion beam, wherein an external magnetic field is appliedto a region of the ion beam, and said second polarity ions areintroduced in said region.